In an organic EL (electroluminescence) device for display devices, emission from a singlet excited state (that is, fluorescence) has been conventionally utilized. In this case, maximum emission efficiency is 25%, and therefore, the emission efficiency is extremely low. As a method of increasing emission efficiency, therefore, utilization of emission from triplet excited state (that is, phosphorescence) has been proposed. When phosphorescence is utilized, the emission efficiency may be 100% in theory.
In addition, a metal complex wherein iridium is cyclometallized with phenylpyridine has been reported to generate phosphorescence at high efficiency even at room temperature. Since then, the study of phosphorescent light emitting materials has been mostly focused on iridium complex as a target, and therefore, the possibility of other metal complexes as a light emitting device has not been sufficiently evaluated yet.
The present inventors tried to synthesize a mixed metal complex by using 3,5-dimethylpyrazole, and succeeded in isolation of a metal complex showing very strong emission on irradiation of UV light. As a result of the measurement of emission from this metal complex, it was found that a mixed metal complex containing platinum and silver exhibited phosphorescent blue emission, and the emission quantum yield in a solid state and a solution was higher than that of a phenylpyridine-iridium complex and 0.85 and 0.51, respectively. The emission properties of this metal complex are not inferior to that of a material having the best emission properties among the compounds known to date as the light emitting materials of organic EL devices.
Moreover, the present inventors have heretofore synthesized a series of mixed metal complexes by using pyrazole having various substituents, and developed some metal complexes having a high solid-state emission quantum yield (see, for example, patent document 1). However, sufficient EL properties have not yet been obtained from conventional metal complexes.